Field of the Invention
This invention relates to the operation of electrostatic precipitators for use in fossil fuel fired power plants which use low sulfur fuels to generate heat or electricity. More particularly, this invention relates to an improved apparatus and process for increasing the particulate removal efficiency of electrostatic precipitators used in such facilities by the conversion of sulfur dioxide, present in the flue gas stream, to sulfur trioxide which reacts with the fly ash to improve the electrical conductivity of the fly ash.
Fuels, typically fossil fuels, are combusted with air in a boiler to generate heat. The heat generated is converted into useful energy to heat a product or process, or to produce electricity. The fossil fuels normally used, such as coal or oil, contained sulfur. When coal is burned, the products of combustion include particulate matter (commonly known as fly ash), sulfur dioxide (SO.sub.2) and water, which are exhausted from the boiler as part of an exhaust stream known as flue gas. Fly ash and SO.sub.2 are both undesirable pollutants and must be removed from the flue gas to a desirable level. These levels are normally set by environmental regulatory agencies.
In most fossil fuel-fired plants, fly ash in the flue gas stream is removed by electrostatic precipitation. An electrostatic charge is applied to the fly ash in the flue gas stream as the flue gas passes between charged electrodes contained in an electrostatic precipitator. The particulate matter is deposited upon the electrode having the opposite charge to that of the fly ash and is later removed. The efficiency with which fly ash is removed from the flue gas stream by the electrostatic precipitator depends in part upon the electrical conductivity of the fly ash. This, in turn, is influenced by the absorption by the particles of fly ash, of sulfuric acid (H.sub.2 SO.sub.4) that is generated as a by-product of the combustion process through the reaction of SO.sub.2 with oxygen and water in the flue gas stream. The sulfuric acid deposited upon the particulate matter imparts a degree of electrical conductivity to the particulate and promotes the electrostatic precipitation process.
When fuels having a relative large sulfur contents are used, only a portion of the SO.sub.2 generated by combustion is converted to the sulfuric acid required for conditioning the fly ash. Absent expensive process equipment for removing SO.sub.2 from the flue gas stream, the excess SO.sub.2 in the flue gas is exhausted to the atmosphere. This is undesirable since SO.sub.2 can cause pollution problems, such as acid rain. One alternative to reduce the amount of SO.sub.2 generated by the combustion process is to use fuel that is lower in sulfur content. However, the combustion of low sulfur coal would also result in the amount of SO.sub.3 produced by the combustion process being insufficient to produce the quantities of sulfuric acid required to efficiently remove fly ash at the electrostatic precipitator. To combat this problem power plant operators have been introducing sulfur trioxide (SO.sub.3) from other sources into the flue gas stream or generating SO.sub.3 by catalytic means within the exhaust system of the power plant.
Archer et al., U.S. Pat. No. 3,993,429 and Hankins et al., U.S. Pat. No. 5,244,642, disclose burning sulfur in air to produce SO.sub.2, which is then catalytically oxidized to SO.sub.3. The SO.sub.3 generated by the processes of Archer and Hankins is directed into the flue gas stream where it reacts with water vapor from the combustion process to form sulfuric acid, which is in turn absorbed by the fly ash. The process of Hankins further provides for a control loop that maintains a constant level of SO.sub.2 produces based upon the fly ash content of the exhaust gas.
Processes that burn sulfur such as Archer and Hankins et al., provide a simple, direct solution to the problem of providing a sufficient amount of SO.sub.3 in a flue gas stream to permit the efficient removal of fly ash by electrostatic precipitation. However, these processes require the addition of extra sulfur to the flue gas exhaust stream which must be removed later in the combustion process by additional pollution control equipment. Further, a separate combustion system must be monitored and maintained. The catalytic system for conversion of SO.sub.2 to SO.sub.3 must also be maintained and replenished with fresh catalysts.
Spokoyny et al., U.S. Pat. No. 5,320,052 describes a sulfur trioxide conditioning system that includes a catalytic converter that oxidizes a portion of the sulfur dioxide in a flow of flue gas to sulfur trioxide. The catalytic converter incorporates a catalyst support, which is disposed across at least a portion of the cross section of a main duct from a burner to a heat recovery apparatus, and a catalyst on the catalyst support. The amount of the catalyst surface exposed to the flow of flue gas is selectively varied to control the conversion of sulfur dioxide to sulfur trioxide. Spokoyny et al., U.S. Pat. No. 5,540,755 discloses an aspirating device to draw SO.sub.2 laden flue gas through the catalyst beds.
The devices of the Spokoyny patents provide a catalytic oxidation system for conversion of SO.sub.2 to SO.sub.3. As with any catalytic system, provisions must be made to compensate for the pressure drop across the catalytic bed. This pressure drop represents a loss of energy that must be made up for with additional fans or blowers. Further, as with all catalytic systems the catalyst becomes deactivated or poisoned with use. The catalyst must be monitored and replenished as needed. This is extremely difficult from a plant operating perspective when the catalyst is located within a transport line as disclosed in the Spokoyny patents.
Alternative catalytic oxidation systems have been disclosed by Altman et al., U.S. Pat. No. 5,011,516, and U.S. Pat. Nos. 5,356,597 and 5,547,495 to Wright et al.
Altman describes an alternate approach in which a flue gas stream is divided into two streams. One flue gas stream is passed through a heat exchanger and continues on to a bag house for particulate removal. The second flue gas stream is passed over a catalyst. A portion of the sulfur dioxide in the second stream is oxidized to sulfur trioxide, and the two streams are then merged back into the main flue gas flow prior to the bag house. The second stream is not passed over a heat exchanger since the gas stream must be maintained at a high temperature to permit efficient conversion of sulfur dioxide to sulfur trioxide. While of interest, this approach has major drawbacks when implemented. System thermal efficiency is reduced because less heat is recovered. Further, there is typically insufficient mixing of the slip stream and the main flow at the point where they rejoin due to an insufficient pressure differential. Moreover, the Altman patent does not disclose any approach which permits control of the amount of sulfur trioxide produced, responsive to variations in the sulfur content of the fuel and changes in other operating parameters.
The Wright patents disclose a catalytic oxidation system in which a catalyst bed is maintained in a slidably mounted platform within the duct connecting the combustion chamber with the electrostatic precipitator. While the devices of Wright provide for the easy removal of the catalyst bed from the flue gas duct for servicing, the problems associated with catalyst deactivation and poisoning is still present.
A system for the simultaneous destruction of SO.sub.2 and NO.sub.x by a plasma reactor has been disclosed by Mathur et al., U.S. Pat. No. 5,020,457. Ammonia, methane, steam, hydrogen, nitrogen or combinations of these gases are subjected to plasma conditions sufficient to create free radicals, ions or excited atoms. These ions or atoms are then reacted with SO.sub.2 and NO.sub.x in the flue gas stream to produce environmentally safe compounds or compounds that can easily be removed from the flue gas stream. In PLASMA-ASSISTED CLEANUP OF FLUE GAS, by S. K. Dhali it is suggested to convert SO.sub.2 and NO.sub.x to stable acid mists which can be removed by mist eliminators.